E. Cobham Brewer 18101897. Dictionary of Phrase and Fable. 1898.
As the dominions of the duke of Saxony were bounded in part by France, one of the early dukes hit upon the device of a coat blue one side, and white the other. When he wished to be thought in the French interest he wore the white outside; otherwise the outside colour was blue. Whence a Saxon was nicknamed Emmanuel Turncoat. (Scots Magazine, October, 1747.)
Without going to history, we have a very palpable etymon in the French tourne-côte (turn-side). (See COAT.)